121 



pyles into the flagellated chambers and from there through the 

 apopyles into the exhalant (excurrent) system, in order to finally 

 leave the sponge body by a large osculum (Fig. 53, 54). 



This current of water is caused by the movements of the 

 flagella of the choanocytes in the flagellated chambers, as is 

 generally acknowledged. The great question which had been dis- 

 cussed for ever such a long time, and which seemed decided in 

 1898 by the research of Vosmaer and Pekelharing (62) — as 

 mentioned in the Introduction — was: what is the exact way in 

 which the flagella move; how is the movement of the water 

 within the flagellated chambers ; how is the whole water-current 

 explained ? 



It is a matter of course that, on account of the difficulty of 

 the research, one has not been able to make many direct obser- 

 vations concerning this question. So Lieberkühn (38) says to 

 have seen the movement of the flagella in Grrantia botryoides 

 (a calcareous sponge), viz. „Wimpern" which „ausserst lebhaft 

 schwingen"; he does not give more details. Bowerbank (7), 

 however, says about the flagella of Grantia compressa: „When 

 in vigorous condition their motions are rapid and cannot readily 

 be followed, but in some in which the action was languid, the 

 upper portion of the cilium was thrown gently backward towards 

 the surface of the sponge, and then lashed briskly forward towards 

 the osculum, and this action was steadily and regularly repeated. 

 Their motions are not synchronous, each evidently acts indepen- 

 dently of the others", v. Lendenfeld (37) says — I quote from 

 Vosmaer and Pekelharing — : „it appears that the cilia in the 

 entodermal collar-cells move, pendulum-like, backward and for- 

 ward, similarly to the cilia of the polyciliar epithelium-cells in 

 the respiratory-tracts and other parts of vertebrates" — a remark, 

 however, apparently not based on observations. Finally Cotte 

 (13): „on peut voir le mouvement des flagella se produire sous 

 forme d'ondes, avec un rythme particulier au moment oü se fait 

 I'observation, mais qui a ce moment est le memo pour tous les 

 flagella d'un même territoire" (so in the way of polyciliar epithe- 

 lium). „Par centre, a cote des cellules a mouvement régulier, on 



